Hot Accretion Flow around Neutron Stars

Abstract

We perform as the first time hydrodynamic simulations to study the properties of hot accretion flow (HAF) around a neutron star (NS). The energy carried by the HAF will eventually be radiated out at the surface of the NS. The emitted photons can propagate inside the HAF and cool the HAF via Comptonization. We find that the Compton cooling can affect the properties of HAF around a NS significantly. We define the Eddington accretion rate asṀ Edd = 10L Edd /c 2 , with L Edd and c being the Eddington luminosity and the speed of light, respectively. We defineṁ as the mass accretion rate at the NS surface in unit ofṀ Edd . Whenṁ > 10 −4 , Compton cooling can effectively cool the HAF and suppress wind. Therefore, the mass accretion rate is almost a constant with radius. The density profile is ρ ∝ r −1.4 . Whenṁ < 10 −4 , the Compton cooling effects become weaker, wind becomes stronger, accretion rate is proportional to r 0.3−0.5 . Consequently, the density profile becomes flatter, ρ ∝ r −1∼−0.8 . Whenṁ < 10 −6 , the Compton cooling effects can be neglected. We find that with a same accretion rate, the temperature of HAF around a NS is significantly lower than that of HAF around a black hole (BH). Also, the Compton y−parameter of HAF around a NS is significantly smaller than that of HAF around a BH. This result predicts that HAF around a NS will produce a softer spectrum compared to HAF around a BH, which is consistent with observations.

A systematic study of the advection-dominated accretion flow for the origin of the X-ray emission in weakly magnetized low-level accreting neutron stars

“…In this paper, a constantṁ as a function of radius is assumed, which has been supported by the recent numerical simulations of the ADAF around a weakly magnetized NS(Bu et al 2019). …”

Two-temperature Radiative Hot Accretion Flow around Neutron Stars

“…Therefore, at a same accretion rate, the electron temperature of a HAF around a NS will be lower than that of a HAF around a BH. This point has been confirmed by analytical (Sunyaev & Titarchuk 1989;Syunyaev et al 1991;Narayan & McClintock 2008;Qiao & Liu 2018) and simulation (Bu, Qiao & Yang 2019) works. The observations to BH-LMXBs and NS-LMXBs do show that electron temperature of a HAF around a NS is lower than that of a HAF around a BH (Burke et al 2017).…”